Capacitor bleed circuit

ABSTRACT

Detecting means detects current flow to a capacitor. When capacitor charging current stops, cessation of charging current is detected, and a quick dump bleed path is connected in parallel to the capacitor.

United States Patent Inventor Appl. No.

Filed Patented Assignee Malcolm M. Oakes Torrance, Calif.

Jan. 8, I969 June 22, 1971 Hughes Aircraft Company Culver City, Calif.

CAPACITOR BLEED CIRCUIT 3 Claims, 4 Drawing Figs.

0.5. CI. 307/246, 307/252 W, 328/67 Int. Cl 03k 17/00 Field of Search 307/246, 252; 328/67 References Cited UNITED STATES PATENTS 3/1956 Clayton 328/67 X 4/1958 White 328/67 X 9/1965 Theodore et al... 328/67 2/l969 Wright 328/67 X Primary ExaminerDonald D. F orrer Assistant Examiner-John Zazworsky Attorneys-lames K. Haskell and Allen A. Dicke, Jr.

ABSTRACT: Detecting means detects current flow to a capacitor. When capacitor charging current stops, cessation of charging current is detected, and a quick dump bleed path' is connected in parallel to the capacitor.

Cont rol ler PATENTFDJUN2219I1 ConiroHer /26 l 54 I 1 i w 24 22 52 3 u? 0 36 5O 44 IO 42 3 4O 48 46 /|4 fl zka H\28 56 I Alg Fig. 2.

Malcolm M. Ookes,

INVENTOR.

ALLEN A DICKE Jr.,

AGENT.

CAPACITOR BLEED CIRCUIT BACKGROUND SUMMARY In order to aid in the understanding of this invention it can be stated in essentially summary form that it is directed to a capacitor bleed circuit. The bleed circuit employs a current detector in the capacitor charging line. As long as this 'detector detects charging current, the bleed circuit is held open. However, upon' cessation of charging current flow, the bleed circuit is closed and most of the charge on the capacitor is quickly dissipated. The bleed circuit remains closed until substantially all charge is dissipated from the capacitor or charging of the capacitor is resumed.

Accordingly, it is an object of this invention to provide a capacitor bleed circuit which quickly bleeds a capacitor upon cessation of capacitor charging. It is another object of this invention to provide a capacitor charge current detector which holds a capacitor bleed circuit open until charging current ceases. It is another object of this invention to provide a capacitor bleed circuit wherein the charge on the capacitor its gate controlled by controller 26 which is controlled by SCR voltage difference between buses 14 and 22, as well as other signals.

These other signals may include charging rate signals, and necessarily include an input indicating the desired charge on the capacitor connected between buses 14 and 22. The controller is connected so that SCR 24 turns on only during the downslope portion of the rectified sine waves. Thus, SCR is only energized as between 90 and 180, and between 270 and 360 of the sine wave. FIG. 4 illustrates an exemplary point at which SCR 24 is made conductive to illustrate that the peak voltages in regulated positive bus 22 with respect to negative bus I4 are controlled.

Negative bus 14 extends through diode 28 and is connected to one side of the primary of pulse transformer 30. Positive bus 22 extends through weld control switch 32 to the other side of the primary of transformer 30. The secondary of transformer is connected to electrodes 36 and 38, at least one of which is movable so that they can both be engaged with workpieces 40 which are to be welded. Weld storage capacitor 42 is connected across buses l4a n d 22 on the side of weld control supplies the energy for closing of the capacitor bleed circuit when capacitor charging ceases.

Other objects and advantages of this invention will become apparent from the study of the following portion of specifica tion, the claims and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 11 is a schematic diagram of a welding power supply circuit employing the capacitor bleed circuit of this invention.

FIG. 2 is a partial schematic showing a portion of an alternative embodiment of the portion of the capacitor bleed circuit.

FIG. 3 is a graph showing the uncontrolled rectifier output, a portion of which is employed to charge the capacitor.

FIG. 4 is an illustrative curve showing the portion of the current employed in charging the capacitor.

DESCRIPTION The capacitor bleed circuit of this invention is generally indicated at 110 in FIG. I. It is illustrated in a welding current supply circuit which employs a capacitor to store energy for welding. Power supply transformer secondary 12 is energized from any suitable power supply primary which is connected to the line through suitable switching and fusing. The transformer secondary normally remains energized through a plurality of welding operations.

Secondary I2 is center-tapped and is connected to negative bus 114. The ends of secondary 12 are connected through rectifier diodes l6 and i8 which have their outputs connected together to unregulated positive bus 20. The waveform of the unregulated power positive bus 20 is illustrated in FIG. 3, and comprises a plurality of positive going half-sine waves. While two rectifying diodes are illustrated, it is clear that a bridge circuit can be employed to provide the current to buses 14 and 20. Full wave rectification is preferred to half wave, for reasons of charging time.

The transfer of power from unregulated positive bus 20 to regulated positive bus 22 is controlled by SCR 24. SCR 24 has switch 32 away from transformer 30. It is this capacitor that is charged to supply the welding energy.

Transistors 44 and 46 are connected in the Darlington configuration. The base of transistor 44 is connected at point 48 to negative bus 14 and is connected through resistor 50 to regulated positive bus 22. The collectors of transistors 44 and 46 are connected through resistor 52 to the regulated positive bus 22 at point 54. The emitter of transistor 46 is connected to the negative bus at point 56 between diode 28 and transformer 30. Capacitor 58 is connected in parallel with diode 28.

During normal operation, secondary 12 is energized and controller 26 regulates SCR 24 so that a selected peak voltage difference occurs between regulated bus 22 and negative bus 14. This peak voltage charges capacitor 42 so that its upper side, as seen in FIG. 1, is positive and its lower side is negative. During this charging operation, the voltage drop due to charging current through diode 28 causes capacitor 58 to charge up to the voltage drop of the diode. The normal voltage drop of such diode is between one-half and 1 volt. Thus, the left side of capacitor 58, as seen in FIG. I, is charged negative with respect to the right side by this voltage drop. For purposes of illustration, the voltage drop will be considered 1 volt. The voltage across capacitor 58 is applied between point 48 and point 56, the base-to-emitter connections of the Darlington amplifier, thus holding the Darlington amplifier in the cutoff state.

The system remains in that condition until loss of energization at transformer secondary 12, or requirement by controller 26 that the peak voltage difference between buses 14 and 22 be reduced. When such occurs, the charge on capacitor 42, when considering positive current flow, flows through resistor 50 to capacitor 58 to make the left side of capacitor 58 more positive with respect to the right side. Assuming a base-to-emitter drop of 1 volt in each of transistors 44 and 46 for exemplary purposes, when point 48 rises to 2-volts positive with respect to point 56, the Darlington connection quickly switches on to provide a capacitor dump path for capacitor 42 through resistor 52 and transistor 46.

Resistor 50 and capacitor 58 are chosen so that it takes a reasonable amount of time delay before transistor 46 turns on. In the example given below, with 60 cycle per second energization of transformer 12, after about three half-cycles of no charging, transistor 46 turns on. Resistor 52 is chosen to permit quick dumping of capacitor 42 consistent with the current limitations of transistor 46. A Darlington connection is chosen for current gain considerations, to quickly turn on transistor 46 to avoid transitional peak power time in transistor 46.

The table of values below gives values of components suitable for employment in a 60-cycle system having a capacitor 42 up to 3,750 microfarads and having a maximum voltage on the capacitor of 400 volts. With these values, and with maximum charge on the capacitor 42, discharge starts about three halfcycles (about msec.) after cessation of energization of secondary 12 or cut off of SCR 24, and substantially complete discharge of capacitor 42 is accomplished within about 3.75 seconds.

60.: Transistor... DTS-40l.

In cases where the transistor 46 is not capable of holding off the maximum voltage on capacitor 42, a voltage divider circuit can be employed as is illustrated in FIG. 2, thereby distributing evenly the state of charge voltage across two transistors instead of one.

The subcircuit of FlG. 2 is connected to points 48, 54 and 56, in FIG. 1, and employs many of the same components. However, in addition transistor 60 is connected between resistor 52 and the collector side of the Darlington connection. The base of transistor 60 is connected between voltage dividing resistances 62 and 64 which divide the voltage between buses 14 and 22. These resistances are preferably both 10 k ohms. Thus, transistor 60 is biased to take half the voltage drop when transistor 46 is shut off.

This invention having been described in its preferred embodiment, it is clear that it is susceptible to numerous modifications and embodiments within the ability of those skilled in the art and without the exercise of the inventive faculty. Ac cordingly, the scope of this invention is defined by the scope of the following claims.

what I claim is:

l. A capacitor bleed circuit, said capacitor bleed circuit comprising:

a capacitor charging circuit and an energy storage capacitor connected thereto to be charged thereby;

capacitor charging detecting means connected between said capacitor charging circuit and said energy storage capacitor, said charging detecting means comprising a resistive device said resistive device being a diode, a control resistor connected between said energy storage capacitor and said diode, a detector capacitor connected in parallel to said diode so that said detector capacitor is charged in one polarity during charging of said energy storage capacitor and is charged in the opposite polarity during noncharging of said energy storage capacitor;

a capacitor discharge circuit, said capacitor discharge circuit being adapted to be connected in parallel to the energy storage capacitor;

said capacitor charging detecting means being connected to said capacitor discharge circuit to turn on said capacitor discharge circuit when said capacitor charging detecting means detects cessation of charging of the energy storage capacitor so that at least a part of the charge on the energy storage capacitor is conducted through said capacitor discharge circuit upon cessation of charging of the energy storage capacitor.

2. The capacitor bleed circuit of claim 1 wherein a switching device is in said capacitor discharge circuit and said switching device comprises a pair of Darlington connected transistors connected across the diode, and further including a bleed control resistor connected in said capacitor discharge circuit to control current flow therein.

3. A capacitor charge and discharge circuit comprising:

a capacitor charging circuit comprising direct current supply means including a source of alternating current, an unregulated DC bus, rectifier means connected between said alternating current source and said unregulated bus, a regulated bus, capacitor charge control means connected between said unre ulated and said regulated bus for controlling the flow 0 direct current from said unregulated to said regulated bus, said charge control means being controllable to select and limit the maximum voltage on said regulated bus, 3. second bus connected to said direct current supply means, an energy storage capacitor connected between said regulated bus and said second bus;

a first energy storage capacitor discharge circuit connected between said regulated bus and said second bus to substantially completely discharge and absorb the energy of said energy storage capacitor when said first capacitor discharge circuit is actuated;

a second energy storage capacitor discharge circuit connected between said regulated bus and said second bus to discharge a portion of the capacitor energy to reduce the voltage on said energy storage capacitor when the voltage on said energy storage capacitor exceeds the peak voltage on said regulated bus, said second capacitor discharge circuit comprising:

capacitor charging detecting means connected to said energy storage capacitor for detecting charging of said energy storage capacitor; and

a capacitor bleed circuit connected between said regulated and second buses and connected to said detector means so that said bleed circuit is turned on to bleed said capacitor when said detector means detects cessation of charging of said capacitor. 

1. A capacitor bleed circuit, said capacitor bleed circuit comprising: a capacitor charging circuit and an energy storage capacitor connected thereto to be charged thereby; capacitor charging detecting means connected between said capacitor charging circuit and said energy storage capacitor, said charging detecting means comprising a resistive device said resistive device being a diode, a control resistor connected between said energy storage capacitor and said diode, a detector capacitor connected in parallel to said diode so that said detector capacitor is charged in one polarity during charging of said energy storage capacitor and is charged in the opposite polarity during noncharging of said energy storage capacitor; a capacitor discharge circuit, said capacitor discharge circuit being adapted to be connected in parallel to the energy storage capacitor; said capacitor charging detecting means being connected to said capacitor discharge circuit to turn on said capacitor discharge circuit when said capacitor charging detecting means detects cessation of charging of the energy storage capacitor so that at least a part of the charge on the energy storage capacitor is conducted through said capacitor discharge circuit upon cessation of charging of the energy storage capacitor.
 2. The capacitor bleed circuit of claim 1 wherein a switching device is in said capacitor discharge circuit and said switching device comprises a pair of Darlington connected transistors connected across the diode, and further including a bleed control resistor connected in said capacitor discharge circuit to control current flow therein.
 3. A capacitor charge and discharge circuit comprising: a capacitor charging circuit comprising direct current supply means including a source of alternating current, an unregulated DC bus, rectifier means connected between said alternating current source and said unregulated bus, a regulated bus, capacitor charge control means connected between said unregulated and said regulated bus for controlling the flow of direct current from said unregulated to said regulated bus, said charge control means being controllable to select and limit the maximum voltage on said regulated bus, a second bus connected to said direct current supply means, an energy storage capacitor connected between said regulated bus and said second bus; a first energy storage capacitor discharge circuit connected between said regulated bus and said second bus to substantially completely discharge and absorb the energy of said energy storage capacitor when said first capacitor discharge circuit is actuated; a second energy storage capacitor discharge circuit connected between said regulated bus and said second bus to discharge a portion of the capacitor energy to reduce the voltage on said energy storage capacitor when the voltage on said energy storage capacitor exceeds the peak voltage on said regulated bus, said second capacitor discharge circuit comprising: capacitor charging detecting means connected to said energy storage capacitor for detecting charging of said energy storage capacitor; and a capacitor bleed circuit connected between said regulated and second buses and connected to said detector means so that said bleed circuit is turned on to bleed said capacitor when said detector means detects cessation of charging of said capacitor. 